In electrostatic printing, after toner has been deposited on a media sheet, the toner must be fused to the media sheet. This action requires that a fuser station heat the toner to a toner fusion temperature (e.g., approximately 190.degree. C.). Toner fusing has been accomplished in a number of ways. One method employs a heating element (e.g., a long, thin light bulb, in some cases) placed inside a rotating metal cylinder. This method has the advantage of temperature stability due to the thermal mass and intrinsic energy reserve of the rotating metal cylinder. Its disadvantage is that it requires considerable energy to initiate and sustain the fusing process. Thus, a substantial "warm-up" period is required before an actual printing/copying operation can commence.
A further prior art fusing apparatus (see FIG. 1) employs a ceramic heating element placed directly over a media sheet fusing path. The ceramic heater is separated from the media sheet by a flexible, tubular belt that rotates at the same rate as a pressure roller disposed below the media sheet. Ceramic heater 10 is separated from media sheet 12 and toner particles 14 by a fuser film cylinder 16. Fuser film cylinder 16 is comprised of a thin polymeric cylinder which is caused to rotate in the direction shown by arrow 18 (by means not shown). A pair of guides 22 cause fuser film cylinder 16 to maintain its cylindrical shape in the region of contact to media sheet 12. A pressure roller 24 forces media sheet 12 (and toner particles 14) against fuser film cylinder 16 and ceramic heater 10 to enable fusing of the toner particles.
The structure shown in FIG. 1 allows ceramic heater 10 to be in pressure contact with toner particles 14 through a very thin thickness of the polymer sheet which comprises fuser film cylinder 16. The rotation of fuser film cylinder 16 prevents smearing of the toner as it passes through the fuser station. Temperature control of ceramic heater 10 is achieved by signals provided by a thermistor 26 resident on ceramic heater 10.
While the structure of FIG. 1 provides an "instant-on" fusing action, the polymeric material which comprises fuser film cylinder 16, tends to tear. Further, friction between the lower-most surface of ceramic heater 10 and fuser film cylinder 16 causes wear of the inner surface of fuser film cylinder 16 and shortens its lifetime. Further, when attempts are made to speed up the fusing process, the temperature of ceramic heater 10 must be raised to assure a proper fusing action. Under such circumstances, the temperature of fuser film cylinder 16 may approach its flow state and destroy the cylinder.
Accordingly, it is an object of this invention to provide an improved fuser structure which enables an instant-on action in an electrophotographic printer/copier.
It is another object of this invention to provide an improved fuser roller structure which enables a high-speed fusing action.
It is yet another object of this invention to provide an improved fuser roller structure which enables both instant-on operation and high speed fusing, while exhibiting high reliability and long lifetime.